The present invention relates to an apparatus and a method for heating plastic preforms. Such apparatus and methods have been known from the prior art for a long time. In these apparatus, plastic preforms produced are usually heated in an oven and are subsequently supplied in this heated condition to a stretch blow machine that expands these plastic preforms into plastic containers such as in particular plastic bottles.
In the course of this, the plastic preforms are fed in these heating devices to a heating section via a distribution star wheel such as for example a sawtooth star wheel. In the course of this, the sawtooth star wheel separates the plastic preforms to set a predetermined pitch between them. Subsequently, these plastic preforms are placed on heating mandrels. These heating mandrels are in turn moved through the heating tunnel by means of a transport chain. The location at which the plastic preform is transferred to the heating mandrel by the sawtooth star wheel, which will be referred to below as the transfer point for the oven infeed, must be adjusted very accurately in order to ensure a problem-free passage of the plastic preforms.
In the case of systems known from the prior art, this sawtooth star wheel is driven via a toothed belt, which toothed belt is in turn coupled with the drive of the transport chain for the heating mandrels. The transfer point is established here once at the factory. In the prior art, if the mouth of the plastic preform is changed, then the pocket size and thus also the transfer point will be accurately adapted via a second sawtooth module. A displacement of this point in the case of a format changeover is consequently possible only by changing the individual module, i.e. the sawtooth star wheel, and only by way of a mechanical post-adjustment or adjustment of the transfer point. However, the latter is not to be recommended for the operator, because this means a time-consuming and error-prone activity.
The coupling between these drives further has the disadvantage that in the case of jamming of the plastic preforms even a “slight” distortion of the clamping sets in the drive train will be detected either not at all or only late. As a result, the transfers of the plastic preforms (possibly also as a result of a displacement of the transfer point) deteriorate and the wear and tear of parts on the heating mandrels and transport spindles will increase unnoticed.
Further, if the mouths of the plastic preforms are located very close to each other in the neck diameter, then these will be moved with one and the same sawtooth star wheel. Thus, part costs will be saved, and above all, the unpopular changes and changeover times at the customer's will be avoided.
The gradation of the diameter change of the plastic preform is approximately 1 mm. As a consequence, the transfer point can, in the worst case, may be displaced by 0.5 mm or, as a result, deteriorate.
In these cases, at least one transfer point will not or never be ideally adjusted. Either one transfer point is adjusted ideally and the other one consequently badly, or none of the two transfer points is adjusted badly but not ideally either.
Although this approach is advantageous in so far as the operator does not need to change the modules, however it has the disadvantage that the transfer accuracy at the oven infeed suffers. As a result, placement errors may more frequently occur, which in turn reduces the transfer quality in general and also to the subsequent machine module.
Also, the wear and tear of the preforms increases as a result of a non-central plunging of the holding mandrel into the plastic preform, which can be detected by an increased PET abrasion in the machine.